merge r3454, r3455, r3486, r3496, r3519, r3523:

merge r3454, r3455, r3486, r3496, r3519, r3523:

SSI fixes:

*) use content type of the parent request in SSI stub block output
   instead of default one
*) SSI %s timefmt has no timezone offset
*) change ngx_http_ssi_filter and ngx_http_charset_filter order
*) do not store an encoded variable value as a new cached variable value
*) fix SSI include stub for valid empty responses
*) allow to use $uid_got in SSI and perl module